1. Field of the Invention
The present invention relates to an antivibration rubber composition and particularly to an antivibration rubber composition used for engine mounts intended for supporting a car engine and suppressing the transmission of vibration
2. Description of the Related Art
In general, an antivibration rubber composition is used for cars for the purpose of reducing vibration and/or noise. The antivibration rubber composition needs to have high stiffness and high strength and also needs to suppress the transmission of vibration. Hence, the value of dynamic magnification (dynamic spring constant (Kd)/static spring constant (Ks)) needs to be small (reduction of dynamic magnification). Conventionally, in order for the reduction of dynamic magnification, for example, carbon black has been used as a reinforcing material and the factors thereof such as the amount, particle size and structure have been controlled. This, however, has been insufficient for the reduction of dynamic magnification. Therefore, such antivibration rubber compositions have been proposed that contain silica instead of carbon black serving as a reinforcing material and therefore have lower dynamic magnification as compared to those containing carbon black (for example, Japanese Patent No. 3233458 and Japanese Unexamined Patent Application Publication No. 2004-168885). Further, from the viewpoint that silica having a large primary particle size (a small BET specific surface area) is effective in the reduction of dynamic magnification, such an antivibration rubber has been proposed that contains 100 parts by weight of a rubber component principally containing natural rubber and 20 to 80 parts by weight of silica having a BET specific surface area of 25 to 100 m2/g and a Δ thermogravimetric reduction of 3.0% or more, the Δ thermogravimetric reduction being defined as a difference between the thermogravimetric reduction at 1,000° C. and the thermogravimetric reduction at 150° C. in thermogravimetric measurement (Japanese Unexamined Patent Application Publication No. 2006_199899).
The antivibration rubber composition disclosed in the Japanese Unexamined Patent Application Publication No. 2006-199899 contains silica having a large primary particle size and therefore is effective in achieving lower dynamic magnification as compared to one containing ordinary silica. The use of silica having a large primary particle size leads to a reduction in the interaction between such silica and rubber, posing a problem in that a rubber vibration isolator has low durability. As described above, there is no antivibration rubber composition having a good balance between durability and low dynamic magnification at present. Therefore, and antivibration rubber composition having a good balance between durability and low dynamic magnification is being demanded.